Tissues deprived of blood and oxygen undergo ischemic necrosis or infarction with possible irreversible organ damage. In some circumstances, however, such as during cardiac surgery, it is desirable to interrupt the normal myocardial contractions (cardioplegia) and actually induce ischemia. Such elective or obligatory ischemia occurs in the presence of safeguards such as cardioplegia-induced cardiac arrest and hypothermia. While these safeguards provide considerable myocardial protection, alteration of myocardial energetics (stunning) and poor postoperative ventricular function still remain significant problems.
Once the flow of blood and oxygen is restored to the organ or tissue (reperfusion), the organ does not immediately return to the normal preischemic state. Reperfused postischemic non-necrotic myocardium is poorly contractile and has reduced concentrations of high energy nucleotides, depressed subcellular organelle function and membrane damage that resolves only slowly. Although reperfusion restores oxygen and reverses ischemia, repletion of high energy nucleotides such as adenosine triphosphate (ATP) and reversal of ischemic membrane damage is slow, and contractile function may be profoundly depressed for a long period. Just minutes of ischemia causes loss of myocardial systolic wall thickening for hours. Longer periods of reversible ischemia may depress contractility for days. Studies confirm that, despite restoration of myocardial flow and a quick recovery of myocardial oxygen consumption (MVO2) following ischemia, there is only very slow recovery of myocardial contractile function. The problems are exacerbated in high risk patients, such as those with poor preoperative ventricular function, recent myocardial infarction or left ventricular hypertrophy. These same problems also occur during organ storage for cardiac transplant, under which there are time constraints due to the limits of myocardial preservation.
Postischemic dysfunction may be due to a variety of factors. Oxygen free radicals may play a role, as generation of free radicals in stunned myocardium has been demonstrated and free radical scavengers have been shown to attenuate contractile dysfunction. Impaired intracellular calcium handling and calcium overload during early reperfusion may contribute to postischemic dysfunction; while calcium infusions enhance contractility in both normal and postischemic myocardium, ischemia as short as a few minutes produces an impairment in sarcoplasmic reticulum calcium transport and a shift of the calcium ATPase activity. Postischemic myocardium is also associated with reduced concentrations of myocardial high-energy phosphates and adenine nucleotides, as obligatory reduction in myocardial ATP content during ischemia occurs as myocytes utilize ATP for maintenance of cellular integrity. Since ATP is essential for myocardial contraction and relaxation, ATP depletion may have detrimental effects upon postischemic myocardial functional recovery.
The high volume of cardiac-related surgeries, both elective and emergency procedures and including cardiac transplants, lead to the above-described problems. Thus, methods and agents to provide protection against myocardial ischemia and to avoid post ischemic dysfunction are needed.